The invention relates generally to the field of electrochromic devices which vary in transmittance in response to an electric field, and more particularly to the art of solid-state electrochromic devices which incorporate polymeric electrolytes.
Conventional electrochromic cells comprise a thin film of a persistent electrochromic material, i.e. a material responsive to the application of an electric field of a given polarity to change from a high-transmittance, non-absorbing state to a lower-transmittance, absorbing or reflecting state and remaining in the lower-transmittance state after the electric field is discontinued, preferably until an electric field of reversed polarity is applied to return the material to the high-transmittance state. The electrochromic film is in ion-conductive contact, preferably direct physical contact, with a layer of ion-conductive material. The ion-conductive material may be solid, liquid or gel. The electrochromic film and ion-conductive layers are disposed between two electrodes.
As a voltage is applied across the two electrodes, ions are conducted through the ion-conducting layer. When the electrode adjacent to the electrochromic film is the cathode, application of an electric field causes darkening of the film. Reversing the polarity causes reversal of the electrochromic properties, and the film reverts to its high transmittance state. Typically, the electrochromic film, e.g. tungsten oxide, is deposited on a glass substrate coated with an electroconductive film such as tin oxide to form one electrode. The counter electrode of the prior art has typically been a carbon-paper structure backed by a similar tin oxide coated glass substrate or a metal plate.
While this conventional electrochromic device structure might be acceptable for data displays in items such as digital watches, it is not suitable for large transparent articles such as windows. While the opaque carbon-paper counter electrode may be replaced with a thin conductive film such as tin oxide, indium oxide or gold, these thin film electrodes encounter lateral electrical resistance which decreases the speed and uniformity of charge distribution as the surface area of the device increases. More importantly, with electric fields of about 1 volt, half-cell reactions which result in the evolution of gas from the electrolysis of water occur at the counter electrode, depending on the polarity, as follows:
______________________________________ Electrode Reaction Standard Potential ______________________________________ Cathode ##STR1## -0.828 volts Anode ##STR2## -1.229 volts ______________________________________
The hydrogen and oxygen gases produced by these reactions form bubbles which impair the optical properties of an electrochromic cell for use as a window.
The use of a metal mesh as the counter electrode is described in U.S. Pat. No. 4,768,865, the disclosure of which is incorporated herein by reference. The invention described therein allows transparency while providing uniform rapid charge distribution over a large surface area and participating in a balancing half-cell reaction at a lower potential which prevents electrolysis of water and concurrent gas evolution which would otherwise occur according to the following reactions, wherein x is typically up to about 0.5: ##STR3##
Instead of the hydrolysis of water at the counter electrode, pictured on the right above, the balancing half-cell reaction in response to the electrochromic transition of tungsten oxide is the oxidation or reduction of the metal of the metal grid counter electrode, which does not produce gas which can form bubbles and decrease the optical quality of the device.
U.S. Pat. No. 4,726,664 to Tada et al. discloses an electrochromic device comprising an oxidation coloring substance coloring in an oxidized state and a reduction coloring substance coloring in a reduced state which are formed on the surfaces of a pair of electrodes, respectively, and an electrolyte held between the two substances, wherein the oxidation coloring substance is a double salt containing an iron hexacyanoferrate and the reduction coloring substance is a tungsten-oxalic acid compound.
U.S. Pat. No. 4,645,307 to Miyamoto et al. discloses an electrochromic device having two electrochromic layers respectively formed on oppositely arranged two electrodes and containing an electrolyte such as a solution of an alkali metal salt in an organic solvent which fills up the gap between the two electrodes. The electrochromic layers are formed of an electrochromic material which can alternately and stably exist in three different oxidation states and assumes three different colors in its respective oxidation states such that there is a clear contrast between the color of this material in its normal or intermediate oxidation state and a composite color given by superposition of the color in the highest oxidation state on the color in the lowest oxidation state. Prussian blue is a preferred example of such an electrochromic material, on condition that an adequate amount of water be present in the electrolyte solution or, alternatively, that the Prussian blue layers be pretreated to substitute alkali metal cation for Fe.sup.3+ interstitially existing in the crystal lattice of Prussian blue.
U.S. Pat. No. 4,773,741 to Inaba et al. discloses an electrochromic display device having a transparent electrode layer coated with a first electrochromic material which takes on color in its electrochemically oxidized state, such as Prussian blue, and an opposite transparent electrode layer coated with a second electrochromic material which takes on color in its reduced state, such as tungsten oxide. For use in initial bleaching or coloration of one of the two electrochromic layers, an auxiliary electrode is disposed in a marginal region of the space between the two opposite electrodes, and an electrolyte occupies the remaining space.
U.S. Pat. No. 4,818,352 to Inaba et al. discloses an electrodeposition method for forming a film of an electrochromically synthesizable and functional substance e.g. Prussian blue, useful as an electrochromic material, on an electrode plate having a conductive coating relatively high in surface resistivity, such as tin dioxide or indium trioxide, by providing the electrode plate with an elongated auxiliary electrode formed of e.g. a metal wire or foil, attached to the outer surface of the conductive coating so as to extend at least along the whole periphery of the electrode plate.
In "Electrochemical and Electrochromic Properties of All Solid-State Tungsten Oxide-Prussian Blue Based Electrochromic Devices" by Oyama et al., Electrochemical Society of Japan, Vol. 57, pp. 1172-1177 (1989), electrochromic devices using polymeric solid electrolytes and incorporating tungsten oxide-Prussian blue are disclosed as being prepared by dispersing homogeneous lithium trifluoromethanesulfonate in a hydrogen-bonding type complex of poly(acrylic acid) and poly(ethylene oxide). A reversible color change between blue and colorless was observed when an appropriate voltage was applied repeatedly to the tungsten oxide and Prussian blue coated electrodes. The coloration and bleaching were found to be controlled with the electrochemical reaction of the tungsten oxide electrode which depends on the injection and disintercalation of lithium ions into and out of the tungsten oxide layer.
In "Spectroelectrochemistry and Electrochemical Preparation Method of Prussian Blue Modified Electrodes" by Itaya et al., Journal of the American Chemical Society, Vol. 104, pp. 4767-4772 (1982), the details of an electrochemical preparation method for Prussian blue are discussed. The electrochemistry of the Prussian blue-modified electrodes is tested in solutions of various supporting electrolytes such as potassium, ammonium and cesium. A spectroelectrochemical study shows the values of molar extinction efficiency and the absorption spectrum of the fully oxidized form of Prussian blue.
In "Electrochromism in the Mixed-Valence Hexacyanides," The Journal of Physical Chemistry, Vol. 85, pp. 1225-1231 (1981), Ellis et al. describe a sacrificial anode method to cause a rapid deposition of Prussian blue thin film. The rate of deposition is dramatically increased by attaching an iron or copper wire to the electrode and inserting it into the solution.
In "Electrochemical Properties of Amorphous Prussian Blue Films Chemically Deposited from Aqueous Solutions" by Yano et al., Proceedings of the Symposium on Electrochromic Materials, 90-2, pp. 125-136 (1990), amorphous Prussian blue films are described as being formed on transparent electrodes by adding hypophosphorous acid to a solution of ferric-ferrocyanide. The method is effective for preparation of a uniform Prussian blue film over a large area. The electrochromic properties of a Prussian blue film are compared with those of crystalline Prussian blue film prepared electrochemically. Application of an electric voltage gave rise to reversible color changes only for Prussian blue in aqueous solution containing lithium, sodium and barium salts.
In "Electrochemical Oxidation and Reduction of Thin Films of Prussian Blue", Journal of the Electrochemical Society, Vol. 125, pp. 886-887 (1978), Neff describes the electrochemical behavior of thin films of Prussian blue, while in "Ion Flux During Electrochemical Charging of Prussian Blue Films", Journal of Electroanalytical Chemistry, Vol. 234, pp. 213-227 (1987), Feldman et al. disclose the incompatibility of Prussian blue with protons.
In "Prussian Blue-Nafion Composite Film and Its Application to a Thin Film Rechargeable Battery," Progress in Batteries and Solar Cells, Vol. 6, pp. 255-256 (1987), Honda et al. describe the preparation and structure of Prussian blue-Nafion composite films. The electrochemical properties are also described, and it is disclosed that Prussian blue-Nafion.RTM. has the interesting property of being an electrochromic material. The color of Prussian blue-Nafion.RTM. is reversibly changed by applying a bias voltage.